Pacing output determination based on selected capture threshold values

ABSTRACT

Approaches for adjusting the pacing energy delivered by a pacemaker are provided. Adjusting the pacing energy involves performing a plurality of capture threshold tests, each capture threshold test measuring a capture threshold of the heart. One or more measured captured thresholds are selected, including at least one capture threshold that is higher relative to other measured capture thresholds acquired by the plurality of capture threshold tests. The pacing energy is adjusted based on the one or more selected capture thresholds.

FIELD OF THE INVENTION

The present invention relates generally to medical systems and, moreparticularly, to adjusting pacing energy of cardiac devices.

BACKGROUND OF THE INVENTION

When functioning normally, the heart produces rhythmic contractions andis capable of pumping blood throughout the body. However, due to diseaseor injury, the heart rhythm may become irregular resulting in diminishedpumping efficiency. Arrhythmia is a general term used to describe heartrhythm irregularities arising from a variety of physical conditions anddisease processes. Cardiac rhythm management systems, such asimplantable pacemakers and cardiac defibrillators, have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically comprise circuitry to sense electrical signals from the heartand a pulse generator for delivering electrical stimulation pulses tothe heart. Leads extending into the patient's heart are connected toelectrodes that contact the myocardium for sensing the heart'selectrical signals and for delivering stimulation pulses to the heart inaccordance with various therapies for treating the arrhythmias.

Cardiac rhythm management systems operate to stimulate the heart tissueadjacent to the electrodes to produce a contraction of the tissue.Pacemakers are cardiac rhythm management systems that deliver a seriesof low energy pace pulses timed to assist the heart in producing acontractile rhythm that maintains cardiac pumping efficiency. Pacepulses may be intermittent or continuous, depending on the needs of thepatient. There exist a number of categories of pacemaker devices, withvarious modes for sensing and pacing one or more heart chambers.

When a pace pulse produces a contraction in the heart tissue, theelectrical cardiac signal following the contraction is denoted thecaptured response (CR). The captured response may include an electricalsignal, denoted the evoked response signal, associated with the heartcontraction, along with a superimposed signal associated with residualpost pace polarization at the electrode-tissue interface. The magnitudeof the residual post pace polarization signal, or pacing artifact, maybe affected by a variety of factors including lead polarization,after-potential from the pace pulse, lead impedance, patient impedance,pace pulse width, and pace pulse amplitude, for example.

A pace pulse must exceed a minimum energy value, or capture threshold,to produce a contraction. It is desirable for a pace pulse to havesufficient energy to stimulate capture of the heart without expendingenergy significantly in excess of the capture threshold. Thus, accuratedetermination of the capture threshold is required for efficient paceenergy management. If the pace pulse energy is too low, the pace pulsesmay not reliably produce a contractile response in the heart and mayresult in ineffective pacing. If the pace pulse energy is too high, thepatient may experience discomfort and the battery life of the devicewill be shorter.

Determination of a capture threshold allows the cardiac rhythmmanagement system to adjust the energy level of pace pulses tocorrespond to the optimum energy expenditure that reliably produces acontraction. The present invention provides methods and systems foradjusting pacing output energy, and offers numerous advantages over theprior art.

SUMMARY OF THE INVENTION

The present invention involves various methods and systems for adjustingthe pacing energy delivered by a pacemaker. In accordance with oneembodiment of the invention, a method of adjusting the pacing energyinvolves performing a plurality of capture threshold tests, each capturethreshold test measuring a capture threshold of the heart. One or moremeasured captured thresholds are selected, including at least onecapture threshold that is higher relative to other measured capturethresholds acquired by the plurality of capture threshold tests. Thepacing energy is adjusted based on the one or more selected capturethresholds.

In accordance with one aspect of the invention, the plurality of capturethreshold tests are performed at predetermined intervals selected tocover the patient's circadian cycle.

One implementation involves selecting the highest capture thresholdacquired by the plurality of capture tests and adjusting the pacingenergy based on the highest capture threshold. The pacing energy may beadjusted to an energy value that promotes patient safety and devicelongevity. In one example, the pacing energy is set to about twice thehighest capture threshold.

In various implementations, the capture threshold tests may comprisesystem-initiated or commanded tests. For example, the pacing energy maybe initialized based on the capture threshold acquired by a commandedcapture threshold test if the commanded test is successful.

According to another aspect of the invention, pacing energy adjustmentprocesses may accommodate operation in several different modes.

Another embodiment of the invention is directed to a pacing energyadjustment system. The pacing energy adjustment system includes capturethreshold testing circuitry configured to perform a plurality of capturethreshold tests, each capture threshold test measuring a capturethreshold of the heart. The system further includes a processor, coupledto the capture threshold testing circuitry. The processor is configuredto select at least one measured captured threshold that is higherrelative to other measured capture thresholds acquired by the pluralityof capture threshold tests. The processor adjusts a pacing energy basedon the at least one selected capture threshold. The system includes amemory configured to store capture thresholds acquired by the pluralityof capture threshold tests.

According to one aspect of the invention, the processor is configured tooperate in several modes including a capture threshold trend only modeand a pacing energy adjust mode.

The processor may be a component of an implantable cardiac rhythmmanagement system. The plurality of capture threshold tests may beinitiated by an implantable device. Alternatively, the plurality ofcapture threshold tests may be initiated by a remote device.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method of adjusting the pacingenergy in accordance with embodiments of the invention;

FIG. 2 illustrates graphs comparing measured capture thresholds topacing energy adjusted in accordance with embodiments of the invention;

FIG. 3 is a flowchart illustrating a method of adjusting the pacingenergy in accordance with embodiments of the invention;

FIG. 4 illustrates a method of initializing the pacing energy based onresults of a commanded threshold test in accordance with embodiments ofthe invention;

FIG. 5 illustrates pacing energy adjustment when the pacing energyadjustment mode is switched to “ON” mode from “Trend” or “Off” modes inaccordance with embodiments of the invention;

FIG. 6 illustrates a cardiac rhythm management system that may be usedto implement pacing energy adjustment in accordance with embodiments ofthe invention; and

FIG. 7 is block diagram of a cardiac pacemaker/defibrillator suitablefor implementing pacing energy adjustment methods of the presentinvention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBOIDMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings which form a part hereof, and inwhich are shown by way of illustration, various embodiments by which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

An implantable pacemaker or other cardiac rhythm management (CRM) devicemay have the capability to determine the capture threshold of one ormore heart chambers. Capture threshold testing may be used by thepacemaker or other CRM device to determine a minimum pacing energyrequired to achieve capture of the chamber or chambers. When thepatient's capture threshold value has been measured, the pacing energymay be adjusted to an energy level that is consistent with commonpractices, patient safety, response to test results, frequency ofcapture threshold testing, and/or transitions between differentthreshold test modes. The present invention is directed to methods andsystems that use selected capture threshold values to adjust the pacingenergy delivered to the heart.

In one example of an automatic capture threshold procedure, thepacemaker delivers a sequence of pacing pulses to the heart and detectsthe cardiac responses to the pace pulses. The energy of the pacingpulses may be decreased in discrete steps until a predetermined numberof loss-of-capture events occur. The pacemaker may increase thestimulation energy in discrete steps until a predetermined number ofcapture events occur to confirm the capture threshold.

Other procedures for implementing capture threshold testing may beutilized. In one example, the pacing energy may be increased in discretesteps until capture is detected. In another example, the pacing energymay be adjusted according to a binomial search pattern.

Capture threshold testing may be performed using various approaches todetermine the cardiac response to pacing. For example, capture thresholdtesting may employ one or more of the approaches described in thefollowing commonly owned U.S. patent applications which are incorporatedherein by reference: Ser. No. 10/733,869, Ser. No. 10,734,599, Ser. No.11/116,578, and 11/116,525.

FIG. 1 illustrates pacing energy adjustment in accordance withembodiments of the invention. The pacing energy adjustment may use dataacquired from system-initiated and/or commanded capture threshold tests.System-initiated threshold tests may be automatically initiated by thepacemaker or by a remote device communicatively coupled to thepacemaker. System-initiated threshold tests may be set up to occur atpredetermined intervals, random intervals, or according to any schedule.Commanded threshold tests refer to tests that are initiated by aphysician or other person. For example, the physician may communicatewith the pacemaker via a device programmer or other remote system toinitiate a capture threshold test.

In accordance with one embodiment, system-initiated capture thresholdtesting is performed at predetermined intervals that are selected toacquire data throughout a patient's circadian rhythm. For example,capture threshold testing may be scheduled to occur every 21 hours, oraccording to another time interval.

A plurality of capture threshold tests are performed 110. Capturethreshold values acquired by the plurality of capture threshold testsare stored in memory. For example, the memory may comprise a bufferconfigured to hold a predetermined number of threshold values. In oneembodiment, the memory buffer is configured to store about 7 capturethreshold values corresponding to about a week of capture thresholdtests, where a capture threshold test is performed about every 21 hours.Each newly acquired capture threshold value is stored in the memorybuffer and the oldest capture threshold value in the memory buffer isdeleted to provide a moving window of capture threshold values. Inalternate embodiments, the memory may store more or fewer than 7 capturethreshold values.

One or more capture threshold values are selected 120. The selected oneor more values are used to adjust 130 the pacing energy delivered to theheart. In accordance with one embodiment, the capture threshold valuesselected to adjust the pacing energy may comprise one or more capturethreshold values that are higher relative to other stored capturethreshold values. The selected capture threshold values may be processedin various ways before setting the pacing energy. For example, theselected higher capture threshold values may be used to compute anaverage value or weighted average value, and the pacing energy may beadjusted based on the average or weighted average value.

In accordance with another embodiment, the highest capture thresholdvalue may be selected and used to adjust the pacing energy. The pacingenergy may be set to the selected capture threshold value (or averagevalue of more than one selected value) plus a safety margin. In oneimplementation, the pacing energy may be set to the selected capturethreshold value or the value computed from multiple selected capturethreshold values plus a safety offset. In another implementation, thepacing energy may be set equal to a multiple of the selected values oraverage value. In one embodiment, the pacing energy is set to abouttwice the highest value of the stored capture threshold values.

The approaches of the present invention may be implemented to adjust thepacing energy such that a satisfactory pacing safety margin ismaintained. For example, adjusting the pacing energy to twice thecapture threshold value provides a 100% safety margin. Further, thepacing energy adjustment may be performed within a range of values toprovide pacing energy output that conforms to pacing device capabilitiesand common practices. For example, a minimum pacing energy level that isconsistent with common practices may be used, e.g., about 2 V. A maximumpacing energy level that is consistent with the intended uses of thepacing device may be used, e.g., about 5 V.

The approaches of the present invention may be used to account forcircadian variation of the patient's capture threshold. In addition,elevations in capture threshold level are responded to relativelyquickly whereas decreases in capture threshold levels are cautiouslyapproached to maintain patient safety.

FIG. 2 is a graph illustrating selection of capture threshold values forpacing energy adjustment in accordance with embodiments of theinvention. In the example illustrated by FIG. 2, the pacing energy isset to twice the highest capture threshold value determined in the pastseven days with a 2 V minimum limit and a 5 V maximum default value.FIG. 2 shows a graph of measured capture threshold values and a graph ofpacing output energies over a 72 day period. As can be seen in FIG. 2,the pacing energy during days 1-16 is set to the minimum pacing outputvalue, which in this example is 2 V, because the measured capturethreshold values remain below 1 V during this period. On day 17, capturethreshold testing acquires a pacing threshold value above 1 V and thepacing energy is adjusted above the 2 V minimum on day 17.

Comparison of the graphs of measured capture thresholds and pacingoutput energies during days 17-26 show that the pacing output energytracks increases in capture threshold. On day 27, an unusually lowcapture threshold is measured. The pacing output does not track thisunusually low capture threshold measurement and responds cautiously to adecreasing trend of capture threshold measurements acquired during days27-38. On day 38, a capture threshold value greater than 2.5 V ismeasured. The system responds by setting the pacing output to thedefault value, which in this example is 5 V. Pacing at the default valueis maintained for 7 days, until the capture threshold value measured onday 38 is deleted from the moving window of stored capture thresholdvalues.

On days 48-53, the capture threshold testing is unsuccessful. After fourconsecutive unsuccessful capture threshold tests, on day 51, the systemresponds by setting the pacing output to the default value. Althoughthis example responds after four consecutive unsuccessful capturethreshold test events, any number may be used. The pacing output energyis maintained at the default value until a successful capture thresholdtest is performed on day 54. The system then continues to track thehighest capture threshold measured within the moving window of the mostrecent 7 successful tests.

FIG. 3 is a flowchart illustrating a method of adjusting the pacingenergy in accordance with embodiments of the invention. The systemperforms 330 a capture threshold test periodically 310, 320, such asonce every 24 hours, once every 21 hours, at random intervals, oraccording to some other schedule. The measured capture threshold valuesare stored in memory. The data is stored so that a moving window ofcapture threshold measurements are maintained, wherein the most recentcapture threshold measurement replaces 340 the oldest capture thresholdmeasurement. If there are less than 7 successful test results stored inthe moving window, all test results are retained. One or more capturethreshold measurements are selected 350 from the stored values and areused 360 to adjust the pacing output.

For example, the moving window may store the test results from the lastseven (or other number of) successful daily tests and the pacing energymay be adjusted based on the highest capture threshold value from thelast seven threshold tests. If seven threshold values are not available,then as many as are available may be used. The default pacing value,e.g., 5V, may be used until a capture threshold test is successfullyperformed. If a predetermined number of successive capture thresholdtests are unsuccessful, the system may flag this and set the pacingenergy output to the default value until a capture threshold test issuccessfully performed.

The seven day moving window, with capture threshold testing performedevery 21 hours, for example, provides protection for variations incapture threshold levels due to circadian changes. Furthermore, the riskof one erroneously low measurement affecting the pacing output energyand putting the patient in danger is reduced. Although a singleerroneous high measurement may increase the pacing level for severaldays, this does not significantly impact device longevity.

The system may accommodate both system-initiated capture thresholdtesting and commanded capture threshold tests. A system-initiatedcapture threshold test may be initiated, for example, by an implantabledevice, such as a cardiac pacemaker. Alternatively, the system-initiatedcapture threshold tests may be initiated by a patient-external system,such as advanced patient management (APM) system in communication withthe implantable pacemaker. The advanced patient management system may beused to automatically initiate tests, and also allows physicians orother personnel to remotely initiate tests and/or monitor patientconditions. In one example, a cardiac pacemaker, defibrillator, or otherdevice, may be equipped with various telecommunications and informationtechnologies that enable real-time data collection, diagnosis, andtreatment of the patient. Various embodiments described herein may beused in connection with advanced patient management. Methods,structures, and/or techniques described herein, which may be adapted toprovide for remote patient/device monitoring, diagnosis, therapy, orother APM related methodologies, may incorporate features of one or moreof the following references: U.S. Pat. Nos. 6,221,011; 6,270,457;6,277,072; 6,280,380; 6,312,378; 6,336,903; 6,358,203; 6,368,284;6,398,728; and 6,440,066, which are hereby incorporated herein byreference.

Capture threshold tests may be performed on command, for example, by aphysician or other person communicating with the implantable pacemakervia a device programmer or APM system. The system's response tosystem-initiated capture threshold tests may be different from thesystem's response to commanded tests. FIG. 4 is a flow chartillustrating a method of adjusting pacing output in accordance withembodiments of the invention when both system-initiated and commandedthreshold tests may be performed. The system initially waits 410 for acapture threshold test to be initiated 415. If the capture thresholdtest is system-initiated 420, the threshold test is performed 425 andthe acquired capture threshold measurement is stored 440 in the movingwindow storage. The highest capture threshold value is selected 445 fromthe stored capture threshold values and is used to adjust 450 the pacingoutput.

If the initiated capture threshold test is 420 a commanded test, e.g.,initiated by a physician or other person, then the system performs 455the commanded test. If the patient's capture threshold is successfullymeasured 460, then the pacing energy is based 465 on the capturethreshold value acquired by the commanded test. The moving window ofthreshold values stored in memory is cleared 470 and the capturethreshold value acquired by the commanded threshold test is stored asthe first value in the moving window. This process avoids the confusionthat would occur during a follow up if the pacing output after acommanded test did not agree with the measurement acquired during thecommanded test as expected.

The pacemaker may have the capability of operating in several differentmodes. For example, the pacemaker may operate in an “OFF” mode, whereinsystem-initiated capture threshold testing is not performed. Thepacemaker may operate in an “ON” mode, where system-initiated capturethreshold testing is performed and selected capture threshold values areused to adjust the pacing output. The pacemaker may also operate in a“TREND” mode, wherein system-initiated capture threshold testing isperformed and the acquired capture threshold values are stored, however,the pacing energy is not adjusted.

The flowchart of FIG. 5 illustrates pacing energy output adjustment 510when the mode is switched to ON from TREND or OFF. The illustratedapproach provides for appropriately utilizing information from previouscommanded and trend-only capture threshold tests.

If there is at least one capture threshold value in the moving windowbuffer that was acquired 520 during a predetermined time interval, e.g.,21 hours, prior to the mode switch, the pacing output is adjusted 540based on the highest stored capture threshold value. The moving windowbuffer may be configured to store about 7 capture threshold values, forexample.

If there is no capture threshold value in the moving window buffer thatwas acquired 520 during the predetermined time interval, the movingwindow memory buffer is cleared 530 and the pacing energy is set to thedefault value, e.g. 5V, until the next successful capture threshold testis performed. The result of the next capture threshold test may beapplied immediately.

The timing of the next system-initiated test is determined by the entrypoint into the 21 hour (or other time value) test interval. For example,assuming a 21 hour test interval, the next system-initiated capturethreshold test may occur anywhere between 1 hour to 22 hours from thetime the system is switched to ON mode. After the pacing output energyadjustment following 550 the mode switch to the ON mode, pacing energyadjustment may continue according to the methods illustrated in theflowcharts of FIG. 3 or FIG. 4.

Referring now to FIG. 6 of the drawings, there is shown a cardiac rhythmmanagement system that may be used to implement pacing energy adjustmentin accordance with the present invention. The cardiac rhythm managementsystem illustrated in FIG. 6 includes a pacemaker/defibrillator 800electrically and physically coupled to a lead system 802. The housingand/or header of the pacemaker/defibrillator 800 may incorporate one ormore electrodes 908, 909 used to provide electrical stimulation energyto the heart and to sense cardiac electrical activity. Thepacemaker/defibrillator 800 may utilize all or a portion of thepacemaker/defibrillator housing as a can electrode 909. Thepacemaker/defibrillator 800 may include an indifferent electrode 908positioned, for example, on the header or the housing of thepacemaker/defibrillator 800. If the pacemaker/defibrillator 800 includesboth a can electrode 909 and an indifferent electrode 908, theelectrodes 908, 909 typically are electrically isolated from each other.

The lead system 802 is used to detect electric cardiac signals producedby the heart 801 and to provide electrical energy to the heart 801 undercertain predetermined conditions to treat cardiac arrhythmias. The leadsystem 802 may include one or more electrodes used for pacing, sensing,and/or defibrillation. In the embodiment shown in FIG. 6, the leadsystem 802 includes an intracardiac right ventricular (RV) lead system804, an intracardiac right atrial (RA) lead system 805, an intracardiacleft ventricular (LV) lead system 806, and an extracardiac left atrial(LA) lead system 808. The lead system 802 of FIG. 6 illustrates oneembodiment that may be used in connection with the pacing outputdetermination methodologies described herein. Other leads and/orelectrodes may additionally or alternatively be used.

The lead system 802 may include intracardiac leads 804, 805, 806implanted in a human body with portions of the intracardiac leads 804,805, 806 inserted into a heart 801. The intracardiac leads 804, 805, 806include various electrodes positionable within the heart for sensingelectrical activity of the heart and for delivering electricalstimulation energy to the heart, for example, pacing pulses and/ordefibrillation shocks to treat various arrhythmias of the heart.

As illustrated in FIG. 6, the lead system 802 may include one or moreextracardiac leads 808 having electrodes, e.g., epicardial electrodes,positioned at locations outside the heart for sensing and pacing one ormore heart chambers.

The right ventricular lead system 804 illustrated in FIG. 6 includes anSVC-coil 816, an RV-coil 814, an RV-ring electrode 811, and an RV-tipelectrode 812. The right ventricular lead system 804 extends through theright atrium 820 and into the right ventricle 819. In particular, theRV-tip electrode 812, RV-ring electrode 811, and RV-coil electrode 814are positioned at appropriate locations within the right ventricle 819for sensing and delivering electrical stimulation pulses to the heart801. The SVC-coil 816 is positioned at an appropriate location withinthe right atrium chamber 820 of the heart 801 or a major vein leading tothe right atrial chamber 820 of the heart 801.

In one configuration, the RV-tip electrode 812 referenced to the canelectrode 909 may be used to implement unipolar pacing and/or sensing inthe right ventricle 819. Bipolar pacing and/or sensing in the rightventricle may be implemented using the RV-tip 812 and RV-ring 811electrodes. In yet another configuration, the RV-ring 811 electrode mayoptionally be omitted, and bipolar pacing and/or sensing may beaccomplished using the RV-tip electrode 812 and the RV-coil 814, forexample. The RV-coil 814 and the SVC-coil 816 are defibrillationelectrodes.

The left ventricular lead 806 includes an LV distal electrode 813 and anLV proximal electrode 817 located at appropriate locations in or aboutthe left ventricle 824 for pacing and/or sensing the left ventricle 824.The left ventricular lead 806 may be guided into the right atrium 820 ofthe heart via the superior vena cava. From the right atrium 820, theleft ventricular lead 806 may be deployed into the coronary sinusostium, the opening of the coronary sinus 850. The lead 806 may beguided through the coronary sinus 850 to a coronary vein of the leftventricle 824. This vein is used as an access pathway for leads to reachthe surfaces of the left ventricle 824 which are not directly accessiblefrom the right side of the heart. Lead placement for the leftventricular lead 806 may be achieved via subclavian vein access and apreformed guiding catheter for insertion of the LV electrodes 813, 817adjacent to the left ventricle.

Unipolar pacing and/or sensing in the left ventricle may be implemented,for example, using the LV distal electrode referenced to the canelectrode 909. The LV distal electrode 813 and the LV proximal electrode817 may be used together as bipolar sense and/or pace electrodes for theleft ventricle. The left ventricular lead 806 and the right ventricularlead 804, in conjunction with the pacemaker/defibrillator 800, may beused to provide cardiac resynchronization therapy such that theventricles of the heart are paced substantially simultaneously, or inphased sequence, to provide enhanced cardiac pumping efficiency forpatients suffering from chronic heart failure.

The right atrial lead 805 includes a RA-tip electrode 856 and an RA-ringelectrode 854 positioned at appropriate locations in the right atrium820 for sensing and pacing the right atrium 820. In one configuration,the RA-tip 856 referenced to the can electrode 909, for example, may beused to provide unipolar pacing and/or sensing in the right atrium 820.In another configuration, the RA-tip electrode 856 and the RA-ringelectrode 854 may be used to provide bipolar pacing and/or sensing.

FIG. 6 illustrates one embodiment of a left atrial lead system 808. Inthis example, the left atrial lead 808 is implemented as an extracardiaclead with LA distal 818 and LA proximal 815 electrodes positioned atappropriate locations outside the heart 801 for sensing and pacing theleft atrium 822. Unipolar pacing and/or sensing of the left atrium maybe accomplished, for example, using the LA distal electrode 818 to thecan 909 pacing vector. The LA proximal 815 and LA distal 818 electrodesmay be used together to implement bipolar pacing and/or sensing of theleft atrium 822.

Referring now to FIG. 7, there is shown a block diagram of a cardiacpacemaker/defibrillator 900 suitable for implementing pacing energyadjustment methods of the present invention. FIG. 7 shows a cardiacpacemaker/defibrillator 900 divided into functional blocks. It isunderstood by those skilled in the art that there exist many possibleconfigurations in which these functional blocks can be arranged. Theexample depicted in FIG. 7 is one possible functional arrangement. Otherarrangements are also possible. For example, more, fewer or differentfunctional blocks may be used to describe a cardiacpacemaker/defibrillator suitable for implementing the methodologies forpacing output determination in accordance with the present invention. Inaddition, although the cardiac pacemaker/defibrillator 900 depicted inFIG. 7 contemplates the use of a programmable microprocessor-based logiccircuit, other circuit implementations may be utilized.

The cardiac pacemaker/defibrillator 900 depicted in FIG. 7 includescircuitry for receiving cardiac signals from a heart and deliveringelectrical stimulation energy to the heart in the form of pacing pulsesor defibrillation shocks. In one embodiment, the circuitry of thecardiac pacemaker/defibrillator 900 is encased and hermetically sealedin a housing 901 suitable for implanting in a human body. Power to thecardiac pacemaker/defibrillator 900 is supplied by an electrochemicalbattery 980. A connector block (not shown) is attached to the housing901 of the cardiac pacemaker/defibrillator 900 to allow for the physicaland electrical attachment of the lead system conductors to the circuitryof the cardiac pacemaker/defibrillator 900.

The cardiac pacemaker/defibrillator 900 may be a programmablemicroprocessor-based system, including a control system 920 and a memory970. The memory 970 provides the moving window buffer for storingcapture threshold values and may also store parameters for variouspacing, defibrillation, and sensing modes, along with other parameters.The memory 970 may be used, for example, for storing historical EGM andtherapy data. The historical data storage may include, for example, dataobtained from long-term patient monitoring used for trending and/orother diagnostic purposes. Historical data, as well as otherinformation, may be transmitted to an external programmer unit 990 asneeded or desired.

The control system 920 and memory 970 may cooperate with othercomponents of the cardiac pacemaker/defibrillator 900 to control theoperations of the cardiac pacemaker/defibrillator 900. The controlsystem depicted in FIG. 7 incorporates a capture threshold testingprocessor 925 for performing capture threshold tests to acquire thepatient's capture threshold. The control system also includes a pacingenergy controller 926. The control system adjusts the pacing energybased on capture threshold values acquired by the threshold testingprocessor in accordance with embodiments of the invention. The controlsystem 920 includes additional functional components including apacemaker control circuit 922, and may include an arrhythmia detector921, along with other components for controlling the operations of thecardiac pacemaker/defibrillator 900.

Telemetry circuitry 960 may be implemented to provide communicationsbetween the cardiac pacemaker/defibrillator 900 and an externalprogrammer unit 990 or other remote system. In one embodiment, thetelemetry circuitry 960 and the programmer unit 990 communicate using awire loop antenna and a radio frequency telemetric link, as is known inthe art, to receive and transmit signals and data between the programmerunit 990 and the telemetry circuitry 960. In this manner, commands, suchas commands relating to threshold testing, may be transferred to thecontrol system 920 of the cardiac pacemaker/defibrillator 900 from theprogrammer unit 990 during and after implant. In addition, storedcardiac data, e.g., pertaining to capture threshold values, along withother data, may be transferred to the programmer unit 990 from thecardiac pacemaker/defibrillator 900. The telemetry circuitry 960 mayprovide for communication between the cardiac pacemaker/defibrillator900 and an APM system as previously described.

In the embodiment of the cardiac pacemaker/defibrillator 900 illustratedin FIG. 7, electrodes RA-tip 856, RA-ring 854, RV-tip 812, RV-ring 811,RV-coil 814, SVC-coil 816, LV distal electrode 813, LV proximalelectrode 817, LA distal electrode 818, LA proximal electrode 815,indifferent electrode 908, and can electrode 909 are coupled through aswitch matrix 910 to sensing circuits 931-937.

A right atrial sensing circuit 931 serves to detect and amplifyelectrical signals from the right atrium of the heart. Bipolar sensingin the right atrium may be implemented, for example, by sensing voltagesdeveloped between the RA-tip 856 and the RA-ring 854. Unipolar sensingmay be implemented, for example, by sensing voltages developed betweenthe RA-tip 856 and the can electrode 909. Outputs from the right atrialsensing circuit are coupled to the control system 920.

A right ventricular sensing circuit 932 serves to detect and amplifyelectrical signals from the right ventricle of the heart. The rightventricular sensing circuit 932 may include, for example, a rightventricular rate channel 933 and a right ventricular shock channel 934.Right ventricular cardiac signals sensed through use of the RV-tip 812electrode are right ventricular near-field signals and are denoted RVrate channel signals. A bipolar RV rate channel signal may be sensed asa voltage developed between the RV-tip 812 and the RV-ring 811.Alternatively, bipolar sensing in the right ventricle may be implementedusing the RV-tip electrode 812 and the RV-coil 814. Unipolar ratechannel sensing in the right ventricle may be implemented, for example,by sensing voltages developed between the RV-tip 812 and the canelectrode 909.

Right ventricular cardiac signals sensed through use of thedefibrillation electrodes are far-field signals, also referred to as RVmorphology or RV shock channel signals. More particularly, a rightventricular shock channel signal may be detected as a voltage developedbetween the RV-coil 814 and the SVC-coil 816. A right ventricular shockchannel signal may also be detected as a voltage developed between theRV-coil 814 and the can electrode 909. In another configuration the canelectrode 909 and the SVC-coil electrode 816 may be electrically shortedand a RV shock channel signal may be detected as the voltage developedbetween the RV-coil 814 and the can electrode 909/SVC-coil 816combination.

Left atrial cardiac signals may be sensed through the use of one or moreleft atrial electrodes 815, 818, which may be configured as epicardialelectrodes. A left atrial sensing circuit 935 serves to detect andamplify electrical signals from the left atrium of the heart. Bipolarsensing and/or pacing in the left atrium may be implemented, forexample, using the LA distal electrode 818 and the LA proximal electrode815. Unipolar sensing and/or pacing of the left atrium may beaccomplished, for example, using the LA distal electrode 818 to canvector 909 or the LA proximal electrode 815 to can vector 909.

A left ventricular sensing circuit 936 serves to detect and amplifyelectrical signals from the left ventricle of the heart. Bipolar sensingin the left ventricle may be implemented, for example, by sensingvoltages developed between the LV distal electrode 813 and the LVproximal electrode 817. Unipolar sensing may be implemented, forexample, by sensing voltages developed between the LV distal electrode813 or the LV proximal electrode 817 and the can electrode 909.

Optionally, an LV coil electrode (not shown) may be inserted into thepatient's cardiac vasculature, e.g., the coronary sinus, adjacent theleft heart. Signals detected using combinations of the LV electrodes,813, 817, LV coil electrode (not shown), and/or can electrodes 909 maybe sensed and amplified by the left ventricular sensing circuitry 936.The output of the left ventricular sensing circuit 936 is coupled to thecontrol system 920.

The outputs of the switching matrix 910 may be operated to coupleselected combinations of electrodes 811, 812, 813, 814, 815, 816, 817,818, 856, 854 to an evoked response sensing circuit 937. The evokedresponse sensing circuit 937 serves to sense and amplify voltagesdeveloped using various combinations of electrodes for discrimination ofvarious cardiac responses to pacing in accordance with embodiments ofthe invention. The capture threshold testing processor 925 may analyzethe output of the evoked response sensing circuit 937 to acquire capturethreshold values. The results of the capture threshold testing may beused by the pacing energy controller 926 to adjust the pacing energy.

Various combinations of pacing and sensing electrodes may be utilized inconnection with pacing and sensing the cardiac signal following the pacepulse to classify the cardiac response to the pacing pulse. For example,in some embodiments, a first electrode combination is used for pacing aheart chamber and a second electrode combination is used to sense thecardiac signal following pacing. In other embodiments, the sameelectrode combination is used for pacing and sensing.

The pacemaker control circuit 922, in combination with pacing circuitryfor the left atrium, right atrium, left ventricle, and right ventricle941, 942, 943, 944, may be implemented to selectively generate anddeliver pacing pulses to the heart using various electrode combinations.The pacing electrode combinations may be used to effect bipolar orunipolar pacing pulses to a heart chamber using one of the pacingvectors as described above. In some implementations, the cardiacpacemaker/defibrillator 900 may include a sensor 961 that is used tosense the patient's hemodynamic need. The timing of the pacing pulsesmay be adjusted to respond to the patient's hemodynamic need based onthe sensor 961 output.

Various modifications and additions can be made to the embodimentsdiscussed hereinabove without departing from the scope of the presentinvention. Accordingly, the scope of the present invention should not belimited by the particular embodiments described above, but should bedefined only by the claims set forth below and equivalents thereof.

1. A method of adjusting a pacing energy delivered to a heart,comprising: performing a plurality of capture threshold tests, eachcapture threshold test measuring a capture threshold of the heart;selecting one or more measured captured thresholds, wherein one of theselected one or more capture thresholds is higher relative to othermeasured capture thresholds of the plurality of capture threshold tests;and adjusting the pacing energy based on the one or more selectedcapture thresholds.
 2. The method of claim 1, wherein performing theplurality of capture threshold tests comprises performing the pluralityof capture threshold tests at predetermined intervals selected to coverthe patient's circadian cycle.
 3. The method of claim 1, whereinselecting the one or more measured capture thresholds comprisesselecting a highest capture threshold acquired by the plurality ofcapture tests.
 4. The method of claim 1, wherein adjusting the pacingenergy comprises adjusting the pacing energy to a highest capturethreshold measured by the plurality of capture threshold tests plus asafety margin.
 5. The method of claim 1, wherein adjusting the pacingenergy comprises adjusting the pacing energy to a predetermined value ifa predetermined number of capture threshold tests are unsuccessful. 6.The method of claim 1, wherein adjusting the pacing energy comprisesresponding to an increasing capture threshold trend more quickly than toa decreasing capture threshold trend.
 7. The method of claim 1, whereinperforming the plurality of capture threshold tests comprises performinga plurality of system-initiated capture threshold tests.
 8. The methodof claim 7, further comprising: storing capture thresholds acquired bythe plurality of system-initiated capture threshold tests; performing acommanded capture threshold test; and clearing the stored capturethresholds and storing a commanded capture threshold test result if thecommanded capture threshold test is successful.
 9. The method of claim8, further comprising adjusting the pacing energy based on the commandedcapture threshold test result.
 10. The method of claim 1, whereinperforming the plurality of capture threshold tests comprises storingresults of the plurality of capture threshold tests in a moving windowbuffer.
 11. The method of claim 1, wherein selecting the one or moremeasured capture thresholds comprises: selecting a stored capturethreshold value, the stored capture threshold value acquired during apredetermined time interval occurring prior to switching to a pacingenergy adjustment mode; and setting the pacing energy to a default valueif there is no stored capture threshold value acquired during thepredetermined time interval.
 12. A pacing energy adjustment system,comprising: capture threshold testing circuitry configured to perform aplurality of capture threshold tests, each capture threshold testmeasuring a capture threshold of the heart; and a processor, coupled tothe capture threshold testing circuitry, the processor configured toselect at least one measured captured threshold that is higher relativeto other measured capture thresholds acquired by the plurality ofcapture threshold tests and to adjust a pacing energy based on the atleast one selected capture threshold.
 13. The system of claim 12,wherein the plurality of capture threshold tests are performed at timeintervals selected to cover a patient's circadian cycle.
 14. The systemof claim 12, wherein the processor is configured to operate in severalmodes including a capture threshold trend mode and a pacing energyadjust mode.
 15. The system of claim 12, wherein the processor isconfigured to set the pacing energy to a default value if apredetermined number of capture threshold tests of the plurality ofcapture threshold tests are unsuccessful.
 16. The system of claim 12,further comprising a moving window buffer configured to store apredetermined number of capture thresholds measured by the plurality ofcapture threshold tests.
 17. The system of claim 16, wherein theprocessor is configured to adjust the pacing energy based on a capturethreshold stored in a moving window buffer.
 18. The system of claim 12,wherein the processor is configured to adjust the pacing energy based ona capture threshold acquired by a commanded capture threshold test. 19.The system of claim 12, wherein the processor is a component of animplantable cardiac rhythm management system.
 20. The system of claim12, wherein the plurality of capture threshold tests are initiated by animplantable device.
 21. The system of claim 12, wherein the plurality ofcapture threshold tests are initiated by a remote device.
 22. A systemfor setting pacing output energy, comprising: capture thresholdcircuitry configured to perform a plurality of capture threshold tests,each capture threshold test measuring a capture threshold of the heart;means for selecting one or more measured captured thresholds, whereinone of the selected one or more capture thresholds is higher relative toother measured capture thresholds of the plurality of capture thresholdtests; and means for adjusting the pacing energy based on the one ormore selected capture thresholds.
 23. The system of claim 22, whereinthe plurality of capture threshold tests are performed at predeterminedintervals selected to cover the patient's circadian cycle.
 24. Thesystem of claim 22, further comprising: means for selecting a highestcapture threshold acquired by the plurality of capture tests; and meansfor adjusting the pacing energy based on the highest capture threshold.